Cellulosic polymers have been made into porous membranes and filters for more than a century. Some control of porosity and pore size of such filters was demonstrated around the turn of the century and absolute, integral filters of specific pore sizes were developed and commercialized in the 1950's. It is a tribute to the early developers and researchers that they were able to make integral filters with pore sizes ranging from about 0.1 to 5.0 microns out of cellulose acetate and cellulose nitrate polymers, essentially the very first synthetic polymers ever produced. These newly developed membrane filters had serious property weaknesses, however, with the primary shortcomings being low tensile strength, poor tear resistance, and excessive brittleness. Altering the polymer chain length or degree of substitution as well as effecting other fundamental changes in the basic polymers had little or no effect on these undesirable properties. Likewise, changes in lacquer composition, process rate, temperatures, or other process environmental conditions had not materially improved or affected these filter qualities. In fact, these filters would probably not have achieved commercial success without post-treatment of the filter to improve flexibility and strength as well as very careful product design of devices incorporating these filters to avoid damage. Flexibilizing agents such as glycerine, and wetting agents such as nonionic detergents have become standard additives to cellulosic filters to give them a broader range of applicability and usefulness. However, these additives are almost always undesirable because they are washed out as the filter is used and they wind up as a contaminant in the filtrate or fluid being analyzed. These agents can be reduced or removed prior to filter use by prewashing the filter, but aside from the inconvenience, doubt will remain as to whether all of the contaminants have been totally removed.
Because of the poor tear strength of the cellulosic membranes, their usage was essentially limited to a single test sample before being discarded. Consequently, researchers turned to stronger nylon membranes to enable multiple test samples to be taken with a single membrane. Nylon microporous membranes and the method for preparing same are well known in the art. Generally, the method for producing these microporous membranes include spreading a coating solution of a polymer containing solvent on a substrate to form a thin film thereon, quenching the film in a bath which includes a non-solvent for the polymer, then removing the membrane film from the substrate. The aforementioned quench technique is described in U.S. Pat. No. 3,876,738 to Marinaccio et al.
Other methods for preparing unsupported filters involve the static gelling of a cast film in a high humidity or controlled atmosphere. For example, U.S. Pat. No. 2,783,894 describes a method for preparing nylon membranes by casting polymer solutions on a smooth substrate to form a film, then exposing the film to an atmosphere of a controlled concentration of nonsolvent vapors for an extended period of time to form a gel structure. The above process suffers from a number of commercial disadvantages, one being the difficulty in controlling the nonsolvent atmosphere in a static environment, the other being that the process does not lend itself to continuous production because of the extended gelation time.
Supported or reinforced microporous membranes suitable for biological fluids have only recently appeared, primarily because the development of such membranes have encountered a number of technical problems including non-uniform wetting of the support web, pinholes and air bubbles in the membrane, among others. These preparation problems would generally result in defects in the membranes, such as voids and air pockets which renders the membrane useless for biological testing purposes. In addition, the pressure drop across the supported membrane tends to be too high.
U.S. Pat. No. 4,645,602 issued Feb. 24, 1987 describes a process for producing a reinforced microporous nylon membrane involving casting successive polymeric solutions onto a support web, then quenching the coated web in a nonsolvent quench bath.
In accordance with the present invention, there is provided a novel process for producing supported microporous cellulosic membrane filters which is relatively rapid, easily controllable and adaptable to continuous commercial production. The supported microporous cellulosic membranes of the present invention maintain the desirable flow and filtration characteristics of unsupported cellulose membranes while providing the superior strength of nylon membranes.